Heterogeneous uptake and oxidation of SO2 on iron oxides

Heterogeneous oxidation of gas-phase SO2 on different iron oxides was investigated in situ using a White cell coupled with Fourier transform infrared spectroscopy (FTIR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results revealed that adsorbed SO2 could be oxidized on the surface of most iron oxides to form a surface sulfate species at ambient temperature. Additional support for this hypothesis was provided by X-ray photoelectron spectroscopy (XPS) measurements and ion chromatogram (IC) analysis. The spectroscopic results further revealed that the surface hydroxyl species on the iron oxides was the key reactant for this heterogeneous oxidation. Furthermore, the bidentate sulfate species were the predominant surface species with alpha-Fe2O3, alpha-FeOOH, and Fe3O4, while the monodentate surface Fe(III)-sulfato complexes were available in the case of gamma-Fe2O3. Using the BET area as the reactive surface area, the samples showed the varied reactivity in the order of alpha-Fe2O3 > gamma-Fe2O3 > Fe3O4 > beta-FeOOH > alpha-FeOOH. Some preliminary experiments indicated a significant acceleration during SO2 uptake on the surface of alpha-Fe2O3 in the presence of oxygen. In contrast, no significant formation of sulfate was seen on the surface of alpha-Fe2O3 prereduced by H-2 at 523 K at the absence of O-2, suggesting that the concentration of adsorbed oxygen over catalyst surfaces may be the key factor contributing to the oxidizing activities. On the basis of these results, the atmospheric implications of these studies on SO2 uptake on Fe-rich mineral aerosol were discussed.